The present invention relates generally to methods and apparatus for chemical engineering analysis, and particularly to a tool for screening chemically reactive systems in assessing their requirements for relief systems.
Over the years, a significant concern in the design and manufacture of chemical process systems and equipment is the provision of what may be termed emergency relief systems. By "emergency relief system" is meant any construction or operational feature of a reactor system whereby excessive pressure build-up may be avoided and venting of the reactor vessel achieved so as to minimize the likelihood of explosion and consequent injury to personnel and/or damage to process equipment.
As in many other fields, a paramount consideration in designing equipment in the chemical engineering field is that such equipment provide safety for operating personnel. In addition, of course, because of the high cost of chemical process equipment, it has become increasingly important to ensure that the equipment will operate safely and without damage to itself in the event of uncontrolled or runaway reactions. The consequences of such accidents depend on their the nature, but range from those which may involve injury to personnel down to those which merely waste or damage some or all of the chemicals being processed.
Aside from the previously mentioned standpoints of avoiding injury to personnel, there has been an increasing concern with the possibility of damage to the environment surrounding chemical manufacturing operations. Environmental damage may range from relatively minor to major damage to downstream or surrounding equipment within a controlled area, damage to the exterior atmosphere, the ground water table, or to buildings or other structures in the near vicinity of the reactor vessel.
Because the nature of chemical processes is not entirely predictable, particularly if there is a human or mechanical malfunction in carrying out the reaction in question, there is both a desire and a need to avoid the damaging consequences of runaway chemical reactions. In some cases, such reactions may create explosions or rapid discharges of vessel contents; in this connection there has been a concern with confining the contents of materials discharged from a reactor vessel to a predetermined area, and where possible, eliminating the conditions which act to aggravate or regenerate the offending condition. In other words, where there is an untoward pressure rise in a reaction vessel, and this pressure or temperature rise a further increase in the reaction rate, immediate steps must be taken to lower the reaction rate or the runaway condition will continue or become worse.
A great deal of effort has been devoted to developing emergency relief systems which may be simply characterized as pressure and/or temperature relief systems which are self actuating in the event of a runaway reaction or other emergency. Creating reactor safety, however, like other industrial concerns, should preferably be achieved with considerations of reliability and low cost in mind. In this connection, it is highly desirable that means be available to generate safe but not overly conservative estimates of the requirements for a relief system. As in other engineering applications, a balance must be struck between a vent or pressure relief system that is so large or oversized that its capacity will never be required, and a system which creates the serious risks of explosion or damage by reason of being undersized.
Preferably, design parameters for relief systems should be able to be established under reduced scale test conditions, so that resort need not be had to creating a full scale, runaway reaction in order to determine vent sizing or other safety measures. A great deal of research in the area of providing proper relief systems has therefore been directed to providing methods and apparatus wherein an accurate scale-up of experimental findings may be made. In other words, where a miniature or small scale test is to be relied upon, it must be established with a high degree of certainty that the small scale reaction under consideration serves as an accurate model for a fully scaled-up reaction.
Referring to another aspect of the need for proper relief system design, because of high capital costs in chemical engineering, and because of the variety of new reactions which are brought on stream in the industry from time to time, it is not always possible to provide a dedicated system of reactor vessels and controls for every reaction which it is desired to carry out on a commercial basis. Therefore, there has been a strong desire in the industry to adapt existing equipment, i.e., equipment designed for a given reaction, for use in other chemical reactions. Each time a new application for the equipment is considered, relief system requirements must be evaluated. In achieving the economies made possible by multiple use of equipment use, however, it is essential that safety considerations not be compromised.
A series of standards and protocols has already been developed for making the calculations necessary to determine whether the venting capacity and mechanism of an existing system is adequate for the type of reaction which it is anticipated will take place in the vessel.
However, while it has been possible to make relatively accurate vent sizing calculations using such test standards and protocols, and to possibly validate such calculations by performing certain of such tests and related experiments, these tests involve the use of unduly expensive equipment and test methods. One such method and apparatus is described and claimed in U.S. Pat. No. 4,670,404. Often, determining proper vent sizing has required using equipment which is very expensive in relation to the economy sought to be achieved by solving the vent sizing problem.
According to the invention, it has been postulated and verified that much simpler test methods and apparatus may be utilized in analyzing various chemical reactions so that what may be termed a screening test may be performed on reactive materials using a screening "tool" for the purpose of determining whether existing relief systems are appropriate. Almost always, the screening tests eliminate the need for expensive and complex studies to validly determine vent size and relief system design.
In accordance with the invention, a reactive system screening tool or apparatus and a method of using it have been developed, whereby performance of simple tests in relatively inexpensive apparatus will enable the user to determine that an existing or proposed relief system design is adequate. The tests involve determining self heat rate or energy release rate for one class of reaction and the gas generation rate for another class of reaction. These values are used in other calculations to determine the adequacy of existing relief systems.
More particularly, according to the invention, it is possible to prepare a small sample of the chemicals which will be used in the full-sized reaction vessel in question, place the sample in the test equipment of the invention and perform a relatively simple analysis in this apparatus.
Thereafter, using the experimental data thus gained, a first formula is applied. Where the reaction system is a vapor or tempered vapor system, this formula provides the energy release rate term applicable to that reaction. In the case where a "gassy" or "hybrid" reaction system is being considered, substantially the same procedure is followed, except that the term developed is the gas generation volume rate term applicable to the reaction.
After the correct values of these rate terms (the "Q" terms) have been determined experimentally, another applicable formula is used to obtain vent size requirement information. This vent size requirement information is then compared to the existing vent size to determine whether the reaction vessel is adequately protected for the anticipated application of the process equipment. If a new vessel is being constructed, the vent system is simply sized from the formula. These formulas and their application are set forth and described in detail elsewhere herein.
Using the apparatus and method of the present invention, it is rarely necessary to resort to full scale testing or even the expensive reduced scale analytical methods of the prior art. The screening tool and method of the invention do not rule out the subsequent use of more complex analytical methods and apparatus where such analysis is indeed necessary, but it has been shown that the need for such complex test may be obviated in a great majority of cases.
Accordingly, it is an object of the present invention to provide a simple laboratory apparatus and method to determine the relief system requirements for various reactive chemical systems.
Another object of the invention is to provide an improved laboratory apparatus which may be constructed of readily available materials at low cost, and yet which will yield reliable test data capable of valid extrapolation for purposes of determining vent sizing or other requirements of a pressure relief system.
A still further object of the invention is to provide a method of operating a simplified apparatus to duplicate existing or projected reactor vessel conditions and thus provide the data necessary to establish or validate relief system requirements.
A further object is to provide a method of relief system sizing which is able to be scaled up with accuracy.
Another object of the invention is to provide a simplified apparatus and method for economically determining whether proposed relief system requirements are adequate, inadequate, or whether further study by more exacting analytical tools is required.
A still further object of the invention is to provide an apparatus which may be manufactured and sold to end users at reasonable costs, and which may be operated to generate relatively accurate, repeatable data useful in predicting actual reaction conditions for a variety of reactions.
Yet another object is to provide a so-called screening tool which employs a novel combination of readily available economical equipment.
Another object of the invention is to provide an apparatus and method which is equally capable of use in so-called vapor, tempered vapor, gassy or hybrid reaction systems.
A further object of the invention is to provide an analytical method which will generate easy-to-interpret date concerning rates of energy released or pressure rise, which data are used to analyze various reactions, including so-called gassy reactions.
A still further object is to provide a screening tool and method which enables determination of relief system adequacy to be made for non-dedicated reaction vessels.
Yet another object is to provide a method of analysis wherein a source term reflecting energy release rate and/or gas generation rate may be found, and wherein this term may be used in determining vent sizing or related relief system data.
The foregoing and other objects and advantages of the invention are achieved in practice by providing an apparatus which includes an exterior containment vessel for providing a controlled atmosphere, a receiver for reactive chemicals disposed in the containment vessel, means for introducing reactants into the vessel from a outside the containment vessel and for controlling or measuring the atmosphere existing within the containment vessel, means for insulating the near environs of the receiver, for heating the contents of the receiver under careful control, and for measuring the temperature of the receiver contents and/or the pressure within the containment vessel, and preferably, means for agitating the chemicals in the receiver; the apparatus preferably also includes appropriate means for reading and, recording the conditions in the receiver and/or the containment vessel.
The method of the invention includes subjecting the contents of a miniature reaction vessel to an atmosphere wherein pressure and temperature may be controlled or observed, measuring the rate of temperature rise in the reactants and/or the rate of pressure increase in the containment vessel to determine the behavior of the reactor contents as conditions change, and using the measured energy release rate or the gas generation rate in certain calculations which take account of the scaled up size of the full scale reaction so that relief system requirements can be established for comparison to existing relief system capabilities provided by existing equipment, or for use in designing new equipment.
The exact manner in which the foregoing and other objects of the invention are achieved in practice will become more clearly apparent when reference is made to the following detailed description of the preferred methods of practicing the invention and the apparatus embodying the invention, which methods and apparatus are set forth by way of example and shown in the accompanying drawings, wherein like references numbers indicate corresponding parts throughout.